Thermotropic liquid crystalline polymers are condensation polymers that have relatively rigid and linear polymer chains so that they melt to form a liquid crystalline phase. The formulations are generally derived from aromatic hydroxy acid monomers (e.g., hydroxybenzoic acid (“HBA”) or 6-hydroxy-2-naphthoic acid (“HNA”)), either alone or in conjunction with other monomers, such as diacids (e.g., terephthalic acid (“TA”) or isophthalic acid (“IA”)) and/or diols (e.g., hydroquinone (“HQ”), acetaminophen (“APAP”), and 4,4′-biphenol (“BP”)). Unfortunately, conventional polymers tend to display a very high solid-to-liquid transition temperature (“melting temperature”), which precludes their ability to be melt processed at temperatures below the decomposition temperature.
To suppress the melting point and generate materials that can be melt processed, additional monomers are often incorporated into the polymer backbone as a repeating unit. One commonly employed melting point suppressant is naphthalene-2,6-dicarboxylic acid (“NDA”), which is generally believed to disrupt the linear nature of the polymer backbone and thereby reduce the melting temperature. The melting point of a liquid crystal polyester may be lowered by substituting NDA for a portion of the terephthalic acid in a polyester of terephthalic acid, hydroquinone and p-hydroxybenzoic acid. Another melting point suppressant is formed from NDA in combination with terephthalic acid and isophthalic acid. NDA has also been employed to help lower the melting temperature of a polyester formed from terephthalic acid, hydroquinone, and optionally 4,4′-biphenol. In addition to NDA, other naphthenic acids have also been employed as a melt point suppressant. For instance, 6-hydroxy-2-naphthoic acid (“HNA”) has been employed as a melting point suppressant for a polyester formed from an aromatic diol and an aromatic dicarboxylic acid.
Despite the benefits achieved, the aforementioned polymers still have various drawbacks. For example, it has been discovered that the heat resistance of such naphthenic acid compositions is relatively poor, as evidenced by a relatively high ratio of deflection temperature under load (“DTUL”) to melting temperature (“Tm”). This is particularly problematic as the demand for heat resistance at high temperatures continually increases in molding, fiber, and film applications.
Various attempts have been thus made to increase this ratio (DTUL/Tm) at lower melting temperatures. For instance, liquid-crystalline polyesters have been formed by solid-state heat treatment of a polymer containing hydroxybenzoic acid (e.g., 4-hydroxybenzoic acid), HNA, hydroquinone, and NDA. While this purportedly addresses one of the problems associated with the use of naphthenic acids (DTUL/Tm ratio), many others still remain. For example, it has been observed that the melting temperature of some liquid crystalline polymers will actually increase when certain naphthenic concentrations (e.g., above 40 mol. %) are reached. This limits the extent that the melting temperature can be suppressed with NDA or HNA. The reactivity of the naphthenic acids with other monomeric constituents is also undesired in many cases and may have unintended consequences on the final mechanical and thermal properties of the polymer composition. In addition to functional concerns, the high cost of naphthenic acids alone dictates that the need for others solutions to the problems noted.
As such, a need continues to exist for a thermotropic liquid crystalline polymer that exhibits a relatively low melting temperature and good heat resistance.